1. Field of the Invention
The present invention relates to a method of coating and heat treating self-piercing rivets (SPR) with a coating that resists galvanic corrosion between steel rivets and aluminum panels joined by the rivets.
2. Background Art
In an effort to improve fuel efficiency and reduce environment pollution, an increasing amount of aluminum has been used for structure and body panels in vehicles. In development of aluminum-intensive vehicles (AIV), self-piercing riveting has received increasing recognition as a potentially effective alternative to spot welding for joining aluminum body panels and structures. Currently, commercially available SPR used for AIV are made predominantly of steel.
When steel rivets are used to join aluminum panels, galvanic corrosion may occur especially if the riveted connection is exposed to moisture. To prevent galvanic corrosion, a sacrificial coating is necessitated. In current commercial practice, the coating is applied to the steel rivets after the completion of forming and heat treating. Rivets are heat treated to obtain an appropriate degree of hardness. The desired hardness depends upon the thickness of the panels to be joined, aluminum alloy grade and other design constraints.
Some existing coating materials adopted by the current SPR manufacturers pose various problems, such as possibly inadequate corrosion performance, instability in paint process, and/or the use of toxic substances for passivation. Thus, an alternative coating material and/or process is needed to eliminate these problems.
One proposed solution is to use a coating known as GEOMET(copyright) that is available from Metal Coatings International, Inc. of Chardon, Ohio, USA, and Dacral, S.A. of Creil, France. GEOMET(copyright) is a chromium-free, water-based VOC compliant coating comprised of overlapping zinc and aluminum flakes in an inorganic binder. Unfortunately, the cure of the coating requires relatively high temperature that may change the mechanical properties of the SPR.
Accordingly, the present invention is directed to the following aspects: (a) eliminating the above problems and, at the same time, (b) achieving the required mechanical properties, (c) improving process efficiency, (d) promoting lean manufacturing and VOC compliance, and (e) enhancing corrosion performance.
According to the present invention, heat treating and surface coating process are combined into a single-step process for low carbon steel SPR applied to join 5xxx and 6xxx aluminum panels. In this single-step process, the conventional temper treatment step of the SPR is eliminated. Only a conventional quenching treatment is performed after completely forming the SPR but prior to coating of the steel rivets. The quenching process is carefully controlled to achieve a hardness level greater than 52 HRc (Rockwell xe2x80x9cCxe2x80x9d scale). In the coating process, a first coat of zinc and aluminum flakes in an inorganic binder is applied to the completely formed and quenched steel rivets. The rivets are subsequently heated to a metal temperature in the range from 350xc2x0 F. to 450xc2x0 F. (or from 177xc2x0 C. to 232xc2x0 C.) for 15 minutes to set the coating. Then, the second coat is applied and cured at a peak metal temperature in the range from 525xc2x0 F. to 600xc2x0 F. (or from 275xc2x0 C. to 316xc2x0 C.) for 15 minutes for final cure of both coats.
Depending upon the design requirements and actual steel grade used, the curing temperatures may vary within the ranges specified above with the curing time being slightly adjusted accordingly.
According to other aspects of the invention, the initial hardness of the steel rivets shall be greater than 52 HRc while the coated rivets, after the single-step heat treating and surface coating process, shall attain a microstructure that corresponds to a hardness level around 47 HRc, that is needed for joining 5xxx and 6xxx aluminum panels.
These and other aspects of the present invention will be apparent to one of ordinary skill in the art in view of the attached detailed description of the preferred embodiments below.